1. Field of the Invention
The present invention relates to a nuclear reactor, in particular a pool-type nuclear reactor cooled with liquid metal or with sodium or with molten salts, having new-concept fuel elements.
2. Description of the Related Art
Many nuclear reactors operating with non-pressurized primary fluid have a pool configuration that includes a core formed by fuel elements, heat exchangers, and circulating pumps, in addition to a series of auxiliary systems and components necessary during normal operation of the plant or during accident situations or during refueling operations. According to known solutions, the core is positioned in the bottom part of the main tank of the reactor, immersed in the primary fluid, and is supported by a supporting grid.
The reactors according to the known solution present various drawbacks.
The core-supporting grid is in general anchored to the bottom of the tank of the reactor and is consequently not replaceable. In the case where the primary fluid is a heavy liquid metal, the fuel elements must also be equipped with a system for coupling to the grid in order to prevent floating thereof. Also the internal structure that has the mechanical function of core containment and of hydraulic separation between the hot manifold and the cold manifold is not replaceable.
In addition, usually positioned above the core are complicated structures for supporting the core control instrumentation, refueling machines comprising rotating plugs, and bars for control of the reactor. In general, there is necessary a large free space around the core for operating with the refueling machines without interfering with the structures for supporting the core control instrumentation.
Since many components are not replaceable, it is necessary to limit damage thereof caused by the neutron flow. For this purpose, each fuel element extends in length underneath the active part in such a way as to reduce damage to the supporting grid. Likewise, between the peripheral part of the core and the internal structure various shielding structures are introduced. The consequence is a significant increase in the size of the reactor.
In addition, handling of the fuel is very complicated because it requires disconnection of cables of the core control instrumentation and of the control bars actuation system to enable movement of the rotating plugs, and because machines are required to be operated within a closed and high-temperature primary system. The risk of blockage of some mechanism and of the lack of cooling of the fuel element during handling is high. Also to prevent this risk, refueling is performed only after the power of decay is sufficiently reduced, by waiting some months from extinction of the reactor or else by setting the exhausted elements to decay in lateral positions with respect to the core. In the first case, however, there is a reduction in the availability of the plant; in the second case, there is an important increase in the size of the reactor.
During operation of the core, fission gases are released by the fuel pellets, there being reserved to said gases, inside the fuel bar, a space comparable to the volume of the fuel. The consequence is a lengthening of the fuel bar, its pressurization to several tens of atmospheres, and the increase in the head losses of the core of the reactor.